Medium-voltage lines (750–34,500 V)
Medium-voltage lines are usually located at the top of utility poles.
They are thin metal wires without an insulated sheath. They are mounted on insulators, which look like small porcelain bowls. These unsheathed lines may look harmless, but they are extremely dangerous!
Medium-voltage lines come in several possible configurations. While there are usually three separate wires, there may be up to six. There may even be a single wire, but don’t be fooled: it’s just as dangerous!
Make sure nobody and nothing—tools, equipment or building materials—ever comes within 3 m of medium-voltage lines.
Why don’t medium-voltage lines have an insulating sheath?
If these are the most dangerous lines, why aren’t they sheathed?
For primarily practical and economic reasons, we rely on the air around them to provide insulation. That’s why they are strung so high, and why it’s so important not to get too close to them or let anything else get too close. If they were sheathed, they would be much bigger and heavier, and bigger or more numerous poles would be needed. The lines themselves would be more noticeable and cost much more. For these reasons, medium-voltage lines are left bare, with no insulating sheath, all over the world.
Low-voltage lines (120–600 V)
Just below the medium-voltage lines are the low-voltage lines. These are the lines that connect houses to the power grid.
The term “low-voltage lines” may make it sound like they aren’t dangerous. But they are. Even 120-V lines can transmit hundreds of amperes, enough to cause serious injury.
These are the two most common configurations of low-voltage lines:
- Two wires with a black insulating sheath, twisted around a bare metal wire
- Three stacked wires with an insulating sheath
Avoid any contact with these lines. Even though they have an insulating sheath, it may be cracked or damaged.
Less often, three bare wires, stacked one above the other, are used for low voltage. NEVER COME WITHIN 3 M OF THEM.
Below the low-voltage lines are large cables covered with a black insulating sheath. These are telecommunications lines, used for telephone and cable service.
Telecommunications lines generally carry 12 V. They don’t belong to Hydro-Québec and aren’t an electrical hazard. But never let anything press on them.
Wondering about those bigger wooden support structures or metal towers?
You’re probably looking at a high-voltage line that transmits power over long distances.
The safety rules about this type of line are even stricter.